TIMING SUBDUCTION PROCESSES VIA IN SITU (IN THIN SECTION) ZIRCON AND BADDELEYITE GEOCHRONOLOGY: EXAMPLES FROM NORTHERN TURKEY

The geology of northern Turkey records the development and destruction of Paleo- and Neo-Tethyan ocean basins as evidenced by mélanges near large-scale suture zones. The Pontide orogenic belt is a ~1500 km section of the Alpine-Himalayan mountain range that accumulated oceanic and continental units during the closure of both ancient oceans. Due to their fragmentary nature, the details of the Paleo-Tethyan closure and related eastern Pontide Orogeny are debated, with regional implications. For example, whether the eastern Pontides formed from northward or southward oceanic subduction is debated. Models that favor a northward subduction suggest the Black Sea is a back-arc basin. Alternatively, southward subduction models the Black Sea as a remnant of Paleo-Tethyan ocean floor. To ascertain which is appropriate, meta-igneous samples collected from the region were studied to determine sources and ages. We find significant trace element differences between metagabbros of the Karakaya (Paleo-Tethyan) and Neo-Tethyan units. Karakaya metagabbros contain blue amphibole, consistent with subduction. Small (2–50 micron) zircon and baddeleyite grains from four Karakaya metagabbros were dated in thin section using an ion microprobe. The results demonstrate the reliability of the method to directly constrain the tectonomagmatic history of these types of assemblages. The rocks yield Late Permian/Early Triassic crystallization ages and an Early Cretaceous minimum metamorphic age. Some zircon and baddeleyite grains with zircon overgrowths yield Early to Middle Jurassic ages. The results constrain a model in which metamorphism and deformation in this region occurred during northward subduction and closure of a Paleo-Tethyan ocean basin and accretion of the Karakaya units to the Laurasian continental margin. This was followed by the onset of closure of the Neo-Tethys during the Campanian-Paleocene and accretion of island arc units to the Tokat region.